1. Field of the Invention
The present invention relates to a switching power supply and, more particularly, to a capacitor charging apparatus for generating high voltage by charging a capacitor.
2. Description of the Related Art
In a variety of electronic equipment, a step-up switching power supply is used to supply a voltage higher than the input voltage to a load. Such a step-up switching power supply, which has a switching element and a transformer, outputs a boosted input voltage by producing a back electromotive force in the transformer by turning the switching element on and off in a time-division manner and charging an output capacitor with the current flowing through the secondary coil of the transformer.
An input voltage is applied to one end of the primary coil of the transformer, and the switching element is coupled to the other end thereof. The voltage at one end of the secondary coil of the transformer is fixed, and an output capacitor is coupled to the other end thereof via a rectifier diode.
With such a switching power supply, when a switching transistor, provided as the switching element, turns on, a current flows through the primary winding of the transformer, and energy is stored in the transformer. Then, as the switching transistor turns off, the energy stored in the transformer is supplied from the secondary side thereof and transferred as a charging current to the output capacitor via the rectifier diode. The output capacitor is thus charged by the repetition of on and off of the switching transistor, and the output voltage rises.
References (1) to (3) listed in the following Related Art List, for example, disclose control circuits for their respective self-excited capacitor charging devices that control the on and off of the switching transistor according to the monitored state of the primary and/or the secondary side of the transformer.